Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In hybrid vehicles, shorter engine operation times can lead to insufficient purging of fuel vapors from the vehicle's emission control system. To address this issue, hybrid vehicles may include a fuel tank isolation valve (FTIV) between a fuel tank and a hydrocarbon canister of the emission system to limit the amount of fuel vapors absorbed in the canister. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of leaks that could release fuel vapors to the atmosphere. This includes diagnosis of leaks on the fuel tank side as well as the canister side of the FTIV. In addition, the mandated orifice size of leaks to be diagnosed on the fuel tank side often vary from the mandated orifice size of leaks to be diagnosed on the canister side. For example, emission control systems may need to be able to detect a 0.020″ orifice on the canister side while also being able to detect a 0.010″ orifice on the fuel tank side.
One example approach for fuel system leak detection is shown by Siddiqui et al. in U.S. Pat. No. 8,074,627. Therein, fuel tank integrity is tested with a fuel tank isolation valve (FTIV) closed by operating a fuel pump to withdraw fuel. A change in fuel tank vacuum is them compared to a threshold to identify fuel tank leakage.
However, the inventors herein have recognized that the approach of Siddiqui et al. identifies a fuel system leak but may not be able to distinguish a leak on the fuel tank side of the FTIV from a leak on the canister side of the FTIV. In addition, the approach of Siddiqui may not be able to distinguish leaks of a first size on a fuel tank side from leaks of different size on the canister side. As such, this may render the system of Siddiqui emissions non-compliant. While the system of Siddiqui may be modified to include reference orifices of different sizes or dedicated leak check modules on both sides of the FTIV, to thereby distinguish fuel tank side leaks from canister side leaks, this may add additional cost and complexity to the system.
Thus in one example, some of the above issues may be addressed by a method for an engine, comprising: indicating leakage on a canister side of a fuel system based on a first change in pressure at a reference orifice following applying of vacuum to the fuel system with an isolation valve closed; and indicating leakage on a fuel tank side of the fuel system based on a second change in pressure at the reference orifice following applying of vacuum to the fuel system with the isolation valve open. In this way, the same leak check module and reference orifice may be used to detect different sized leaks on either side of a fuel system.
For example, a fuel system coupled in a hybrid vehicle may be configured with an evaporative leak check module (ELCM) including a vacuum pump, a reference orifice, and a pressure sensor. The leak check module may be coupled into a vapor line of the fuel system via a three-way switching valve. In particular, the leak check module may be coupled to a fuel system canister along a vent via the switching valve, the valve further plumbed so as to be selectively coupled to the fuel tank vapor line. The fuel tank vapor line may couple the fuel tank to the canister via a fuel tank isolation valve (FTIV) so that refueling vapors generated in the fuel tank can be stored in the canister and diurnal vapors can be held in the fuel tank. A position of the switching valve may be adjusted during leak tests so as to selectively apply vacuum from the ELCM on either the canister side or the fuel tank side of the fuel system. For example, the ELCM may be operated with the switching valve in a first position, and with the FTIV closed, so as to apply vacuum from the ELCM onto the canister side. During this mode, the ELCM may be directly communicating with the canister and not with the fuel tank. A first change in pressure may be monitored at the reference orifice following the applying of vacuum. If the first change in pressure is higher than a first threshold, a fuel system leak on the canister side may be confirmed. The ELCM may then be operated with the switching valve in a second position, and with the FTIV open. During this mode, the ELCM may be directly communicating with the fuel tank and not with the canister. A second change in pressure may be monitored at the reference orifice of the leak check module following the applying of vacuum. If the second change in pressure is higher than a second threshold, a fuel system leak on the fuel tank side may be confirmed.
In this way, each of a fuel tank side and a canister side of a fuel system may be diagnosed using the same leak check module, including the same vacuum pump and reference orifice. By adjusting the position of a switching valve coupling an inlet of the leak check module with the vapor line between the fuel tank and canister, the leak check module can be selectively coupled to only the canister or only the fuel tank. This reduces the need for additional components to diagnose and distinguish leaks on both sides of a fuel system. By comparing the change in pressure estimated on either side of the fuel system to different thresholds, the same reference orifice can be used to diagnose leaks of different sizes on the canister side relative to the fuel tank side. The same may alternatively be achieved by applying a different amount of vacuum on the canister when the fuel tank is isolated, as compared to the amount of vacuum applied on the fuel tank when the canister side is isolated. In this way, leaks of different sizes on different sides of a fuel system may be better identified and distinguished. Overall, emissions compliance may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.